There seems to be some sort of Rousseau-ean idea that if one just returns to some Eden of original movement one has "aiki".

Feldenkrais is an incredible system that will re-educate the body / mind in very deep ways. But I can guarantee you that an attack by someone with a committed attack will produce tension that is the result from fear. Just because the person is relaxed and moves naturally, doesn't mean he has done anything other than attain the pre-requisite for doing technique with "aiki". The ability to maintain that relaxation under pressure is a specialized skill.

Doran Sensei once told me that he had occasion to do training for some advanced Zen students. He was amused to find that these students, who had all the meditation and probably had the spiritual insights on a deep level, had all the same issues which a typical beginner had in terms of how to use the body.

"Aiki" in terms of technique REQUIRES relaxation of the mind and body but that doesn't mean that that's all there is to it. Aiki has to do with how and where you project your attention. It has to do with very subtle movement at the instant of contact which involves a great deal of sensitivity and an understanding of the geometry involved that one simply doesn't have without extensive training.

A highly trained practitioner of an "aiki" art will execute technique in what looks like a very natural manner. But a person who simply moves naturally, no matter how relaxed, cannot use "aiki" without the systematic application of other principles. There are teachers around like Don Angier Sensei of the Yanagi Ryu who have done a very good job of enumerating these principles. I maintain that the mind and body in no way will react to stress using these principles without a lot of re-programming and study of the specific principles involved. Other activities like meditation, Yoga, body work, etc can be of benefit when added to proper training but none of them in themselves will to anything at all for you in terms of understanding "aiki" in the martial context.